Iterator Design Pattern

Motivation
One of the most common data structures in software development is what is generic called a collection. A collection is just a grouping of some objects. They can have the same type or they can be all cast to a base type like object. A collection can be a list, an array, a tree and the examples can continue.
But what is more important is that a collection should provide a way to access its elements without exposing its internal structure. We should have a mechanism to traverse in the same way a list or an array. It doesn't matter how they are internally represented.
The idea of the iterator pattern is to take the responsibility of accessing and passing trough the objects of the collection and put it in the iterator object. The iterator object will maintain the state of the iteration, keeping track of the current item and having a way of identifying what elements are next to be iterated.

Intent
Provide a way to access the elements of an aggregate object sequentially without exposing its underlying representation.
The abstraction provided by the iterator pattern allows you to modify the collection implementation without making any changes outside of collection. It enables you to create a general purpose GUI component that will be able to iterate through any collection of the application.

Applicability
Use the Iterator pattern
1. to access an aggregate object's contents without exposing its internal representation.
2. to support multiple traversals of aggregate objects.
3. to provide a uniform interface for traversing different aggregate structures (that is, to support polymorphic iteration).

Benefit(s)
1. It supports variations in the traversal of an aggregate.
2. Iterators simplify the Aggregate interface.
3. More than one traversal can be pending on an aggregate.

Happy Learning.